Chromatic dispersion involves the spreading of an optical signal due to the wavelength dependence of the velocity of propagation of the optical signal. If an optical signal contains multiple wavelengths, chromatic dispersion can cause the constituent wavelengths of the optical signal to travel at different speeds through the optical fiber so that they arrive at different times at the receiver, resulting in a “spreading” of the optical signal. Chromatic dispersion may occur as a result of the materials within the optical fibers and/or geometries of the optical fibers.
Excessive amounts of accumulated dispersion in high-speed optical communication systems can severely degrade the quality of the transmitted signals. The effect of accumulated dispersion is particularly important in communication systems that transmit signals over long spans of standard single-mode fibers, which may exhibit chromatic dispersion of up to 17 ps/nm/km. To reduce the resulting signal degradation, the chromatic dispersion in the fiber link is often compensated for using dispersion-compensating modules that are interspersed with the fiber spans and designed to substantially reduce the accumulated total chromatic dispersion in each fiber span. However, the exact amount of accumulated dispersion, which transmitted signals experience in the fiber spans and dispersion-compensating modules in a given link, is often unknown because the fiber link was originally designed to transmit signals at substantially lower data rates, which are more tolerant to residual accumulated chromatic dispersion. Therefore, in order to assess whether a certain transmission link can be upgraded to transmit signals at higher data rates, the overall accumulated chromatic dispersion of the fiber link must be re-measured.
In general, chromatic dispersion measurement can be classified into the categories of time-of-fight (TOF), modular phase shift (MPS), and optical-interferometry based methods. (See, H. Chi and J. Yao, “Fiber chromatic dispersion measurement based on wavelength-to-time mapping using a femtosecond pulse laser and an optical comb filter”, Optics Communications 280 (2007) 337-342, which is incorporated herein by reference.)
The chromatic dispersion can be measured either individually for each fiber span and dispersion-compensating module or, alternatively, in a single end-to-end measurement. Most existing dispersion measurement devices utilize a single frequency tunable laser and require a single measurement for each measurement frequency step.